Process of heat treating metals



Patented 21,- 1936 UNITED STATES PATENT 1 OFFICE 2,038,208 PROCESS OF TREATING METALS William E. Day, Jr.,

Plainfield, N. J., assignor to International Motor Company, New York, N. -Y., a corporation of Delaware No Drawing. Application March 2a, 1933, Serial No. 662,295

I 4 Claims.

The present invention relates to methods for heat treatment of axles, shafts, etc., and embodies, more specifically, an'improved method of to the relative shear and tensile properties in the material along the lines in which these stresses lie. In shafts which are. produced either by rolling or forging a weakness exists in the direction of the axis due to the preferential orientation of slip planes (the planes represented by the less densely packed atoms in the primary crystal). The 'flow lines, in general, after severe working, follow the direction of the slip planes. These lines, however, cannot be destroyed by any sort of treatment whereas the slip planes may be treatment described herein, to

this structure broken down ing. This weakness in shafts may be intensified by certain heat treatments and eliminated by others. In general, treatments producing, extreme hardness aggravate this condition.

of the metal cannot be completely By the method described herein, the preferential orientation of slip planes is definitely elimi nated and the section of the shaft adjacent'the surface is treated to produce extreme fitness therein withno brittleness.

high strength but neverthelesspossessed of ductility and free from brittleness.

The strength of steel in torsion member's, ticularly in shafts heat treated by methods now in practice, does not exceed 160,000 pounds per square inch and in most cases is considerably less than this value. This refersv to the maximum shear stress which is taken as the maximum torsional moment over the polar section modulus.

par-

AJl attempts to increase this value by heat treat-' ing to greater hardness have failed because the material develops directional weakness when so treated by conventional methodsfthe shaft rupturing by splintering frequently'at extremely low values. i

In accordance with the heat treatment of the present invention, the directional weakness is even after long periods of anneal-- The treatment further produces a shaft having a, core of relatively overcome and the permissible shear stress is raised to values exceeding 200,000 'pounds per square inch. Furthermore, the method in accordance with the present invention produces a. strength gradient which is a maximumat the 5 outer fibre of the shaft, thus producing a condition which is in accord with the theory noted above as to shafts in torsion. Shafts which have been treated in accordance with the present invention frequently require three times more work 10 to rupture them in torsion than is' the case with shafts which aretreated in accordance with common practice.

The method comprised by the present invention is particularly applicable in the treatment of 15 shafts made from steels of analyses such that when subjected to a hardening heat treatment and tempered they will possess a martensitic or martensito troostitic structure, and, more particularly, those alloy steels known as pearlito martensitic steels; of which the chrome-nickel steel referred to hereinafter is a representative example.

According to the method of the present invention,'a. 'steel shaft of a suitable analysis is heated 25 until the temperature throughout is slightly in excess of its upper critical point ,(hardening point). The shaft is then quenched. The shaft, which has now been hardened throughout, is then tempered by reheating to a tempering temperature and quenching from such temperature, the tempering temperature being such as to retain the steel in a martensitic state; After this preliminary treatment, the shaft is heated rapidly until the temperature in the surface portions has been brought above the upper critical pointwhile controlling the rate of heat input and time of heating so that the core is heated to a temperature slightly below the upper critical point, thereby insuring that the carbides in the surface layer 40 are substantially completely absorbed by the gamma ironand the carbides in the adjoining layers are more or .less completely so absorbed. The shaft is then quenched. The time of heating and the temperature of theheating medium must be consistent with the section and analysis of the material undergoingtreatment. After the -f0regoing operation, the shaftis drawn byre-' F. and 50 heating to a temperature between 300 400 to remove strains A specific illustration of this method is as follows:

A shaft 36" long and 1.6875" in diameter throughout its body, having its endssplined for a distance of 3.5" and of a diameter-at this 55 point of slightly over 2", consisting of material comprising 3.5% nickel, 1.30% chromium, 35% carbon, .02% phosphorus, .02% sulphur, and the balance of iron, is first properly annealed and then heated to 1425 F. The shaft is quenched in oil and reheated to 800 F. and quenched in water. The foregoing steps are in accordance with comon practice. The shaft is then immersed in a lead bath for seconds, the temperature of the bath being 1800 F. It is then quenched in oil and drawn at 425 F. The time of heatin in the 1800 F. bath for shafts of'larger diameter must necessarily be increased. For example, a shaft of 1.9" in diameter must be heated for seconds in accordance with the factors outlined hereinafter.

To determine the proper length of time during which a shaft is subjected to the lead bath, in the case of a circular shaft, the following differential equation, given by Carslaw in The Conduction of Hea at page 222, is solved, the temperature at any point from the axis of the shaft being dependent upon its'radius and the time factor.

K sake?) Pl; bt The solution of this quation is given by Carslaw as; I

M at] 1 jok n E. n

In the foregoing equation:

V=temp. at a Vu=temp. at surface r=a a=radius k=constant of diffusivity t=time The summation is taken over the positive roots of ]0( In ordinary problems of heat diffusion where K is the conductivity and c and p the specific heat and density, respectively. In medium carbon steel the numerical value of k==.121 C. G. S. units. The use of this value is permissible up to a point where the critical range of the steel isreached. At thispoint a sharp change in the specific heat takes place with a corresponding change of heat absorption due to the change of the metal from the alpha to the gamma state. The mean value of k as determined under this condition through experimental means is found to be .0135. The use of this value in the above equation together with the other proper substitutions, and then solving for it gives The foregoing solution is accurate where the heating time is such that the critical point of the acteristics of shafts treated. in accordance with the present invention and affords a comparison between the shafts produced by the old method and those produced in accordance with the present invention.

Torsional moment Old shaft New shaft 1. At elastic 115, 0O0# 120, 000# 2. At ultimate 152, 000- 0,

156, 000i? 210, 000i? 3. Angle at elastic limit 21 4 4. Angle at ultimate strength 180, 200 400-500 It will thus be seen that shafts treated in accordance with the present invention have at least 25% more torsional capacity and are able to resist at least twice as much shock as shafts treated in accordance with the present methods.

Although the foregoing has referred to the treatment of shafts,.the method of the present invention is well suited to the hardening of springs and other elements subjected to bending by reason of the high strength produced in the surface layers by the improved method and the lack of brittleness of the said layers.

While the invention has been described in connection with the heat treatment of a shaft having the characteristics noted herein, it is not to be limited, either as to the nature of the objects treated, nor as to the specific method of treatment. save as defined in the appended claims.

I claim as my invention:-

L-The process of heat treating structural steel parts made from steels of the pearlito-martensitic type to increase their resistance to impact and torsional stresses which comprises subjecting such a part to a hardening heat treatment wherein the said shaft throughout its section is heated above the upper critical range of the steel and then quenched, then subjecting said part to a tempering heat treatment followed by quenching to place the steel in a predominantly martensitic state throughout, heating the thus treated part to bring the temperature in its surface portions above the upper critical range while controlling the rate of heat input and time of heating so as to create a temperature gradient inwardly from the outside of the part such that the core and the innermost layers are heated slightly below the upper critical range, and then quenching.

2. The process of heat treating steel shafts and the like made from steels of the pearlito-martensitic type to increase their resistance to torsional stresses which comprises subjecting such a shaft to a hardening heat treatment wherein the said shaft throughout its section is heated above the upper critical range of the steel and then quenched, then subjecting said shaft to a tempering heat treatment followed by quenching to place the steel in a predominantly martensitic state throughout, heating the thus treated shaft to bring the temperature thereof in its face portions abovethe uppercritical range whilepromoting a aosaaoa i surcontroliing the rate of heat input and time of heating so as to create a temperature gradient along the radii of the shaft such that the core and the innermost layers are heated slightly below the upper critical range, and then quenching, thereby hardening the outer layers while tempering .of the core and inner layers. 7

3. The process of heat treating steel shafts and the like made from a pearlito-martensitic chrome-nickel steel toincrease their resistance to torsional stresses which comprises subjecting such a shaft to a hardening heat treatment wherein the said shaft throughout its section is heated.

above the upper critical range of the steel and then quenched, then subjecting saidshaft to a tempering heat treatment followed by quenching,

heating the thus treated, shaft in a lead bath until its surface portionshave been brought above the upper critical range, and then, while the temperature of the core is still below the upper critical range, quenching the shaft in oil,.and

subjecting the same finally reheating to a moderate temperature below the lower critical range to remove strains.

4. As an article of manufacture, a heat treated structural'steel part made from a pearlito-martensitic steel and having a hardened and tempered core in supporting relation to an overlying hardened outer portion, said part being made by to a hardening heat treatment wherein the saidshaft throughout its section is heated throughout the upper critical range of the steel and then quenched, then subjecting said part to a tempering heat treatment followed byquenching to place the steel in a pearlitomartensitic state throughout, heating the treated part to bring the temperature in its surface portions above the upper critical range while controlling the. rate of heat input and time of heating so as to create a temperature gradient inwardly from the outward side of the part so that the core and the innermost layers are heated to the upper critical range of the steel, and then quenching. E. DAY, JR. 

